Generally, construction machines like hydraulic excavators, for example, are mounted on a crawler type vehicle or lower structure to ensure stabilized travels on mountainous or swampy terrains or other rough terrains.
As well known in the art (e.g., from Japanese Laid-Open Patent Application H9-142337), this sort of crawler type lower structures by prior art are equipped with a truck frame, including a center frame on which an upper structure is to be mounted, a left side frame located on the left side of the center frame and extended longitudinally toward front and rear ends, a right side frame located on the right side of the center frame and similarly extended longitudinally toward front and rear ends, and fore and hind legs which are arranged to connect the right and left side frames with the center frame.
In the case of the prior art just mentioned, a round girth member is provided on the center frame, and a left fore leg, a right fore leg, a left hind leg and a right hind leg to be extended in the shape of letter “X” from the round girth member are connected between the center frame and the right and left side frames. In this instance, the left fore leg is arranged in the form of a box-like structure, by the use of upper and lower plates which are extended from the center frame toward the left side frame in vertically confronting relation with each other, and front and rear plates which are located on the front and rear sides of the upper and lower plates and joined with the latter by welding along respective upper and lower side edges. Similarly to the left fore leg, the right fore leg, left hind leg, right hind leg are formed in the form of a box-like structure which is enclosed by upper, lower, front and rear plates on four sides.
However, in the case of the truck frame of the prior art mentioned above, the four legs are each constituted by four separate plate members, namely, by upper, lower, front and rear plate members. Therefore, in the course of fabricating the legs of the truck frame, it has been inevitably required to cut upper, lower, front and rear plates into specified shapes in the first place and then weld the front and rear plates to the upper and lower plates. These plate cutting and welding operations are required for each one of four leg portions, and are therefore necessarily reflected by an increased truck frame production cost.
Further, at the time of welding to the side frames the four leg portions which are provided integrally with the center frame, a fore end portion of each leg has to be cut off in preparation for forming a gapless butt joint between the fore end of the leg portion and a joint surface on the side of the side frame.
However, it is often the case that joint portions which are formed by welding fore ends of leg portions to a side frame after a cutting operation are found to be inferior in joint strength due to existence of gaps or interstices which are attributable to errors in the course of cutting operations.